End labeling studies of fragmented DNA in the avian growth plate: evidence of apoptosis in terminally differentiated chondrocytes

The chondro-osseous junction has been the subject of considerable scrutiny, especially in terms of the fate and role of the terminally differentiated chondrocyte. Although it has been proposed that these cells change their phenotype and survive in the epiphysis, possibly as osteoblasts, evidence from a number of other studies suggests that chondrocytes may undergo apoptosis or programmed cell death. A useful test for programmed cell death is to end label DNA in cryosections using the commercial reagent ApopTag and detect antibody binding to fragmented DNA by epifluorescence; more direct assessments include examination of the nucleus for condensation of chromatin evaluating fragmentation through alkaline and pulsed field agarose gel electrophoresis of DNA, and measuring apoptosis by flow cytometry. We found that we could label cells in the proliferative and the hypertrophic region of the proximal tibial growth plate of the chick with ApopTag. Most of the chondrocytes in the hypertrophic region were labeled by the reagent; in contrast, few proliferative chondrocytes were stained by the end-labeling procedure. Both agarose and pulsed field electrophoresis were used to confirm that there was fragmentation of chondrocyte DNA. Alkaline gel electrophoresis indicated that there was more fragmentation of DNA from hypertrophic cells than from proliferative chondrocytes. Further evidence in support of apoptosis was provided by electron microscopic observation of cells in the hypertrophic region of the growth plate. We noted that many of the cells in this region of the growth plate appeared to be undergoing programmed cell death since their nuclei contained condensed chromatin. Finally, we used flow cytometry to analyze chondrocytes isolated from the proliferating and hypertrophic regions of the growth plate for apoptosis. Dual parameteric flow cytometric contour plots of Hoechst and 7-amino-actinomycin D fluorescence showed that abut 8% of cells in the plate were apoptotic. Most of these cells were in hypertrophic cartilage. In summary, the results of this investigation indicate that chondrocytes terminate their life history by apoptosis. While it is possible that the terminal labeling studies may overestimate the number of cells undergoing this event, the data lend credence to the view that cells are removed from the epiphysis through apoptosis. If this is the case, then chondrocytes probably enter the terminal phase of their life as fully functioning cells and genomic, and/or local environmental conditions provide termination signals that initiate events that lead to programmed cell death.

Ultrastructural localization of collagen types II, IX, and XI in the growth plate of human rib and fetal bovine epiphyseal cartilage: type XI collagen is restricted to thin fibrils

The collagen fibrils of hyaline cartilage vary in diameter depending on developmental stage and location within the tissue. In general, growth plates and fetal epiphyseal cartilages contain fibrils with diameters of less than approximately 25 nm, whereas the permanent cartilage of adult tissues contains fibrils of approximately 30-200 nm. The interstitial collagen fibrils of fetal cartilage are complex, having at least three collagen types as integral components. Type XI, a member of the fibrillar collagen class, has been proposed to limit fibril diameter. To test this proposition we sought to determine if Type XI collagen was preferentially associated with fibrils of smaller diameter. We focused our study on human juvenile rib growth plate, which has thin fibrils in the hypertrophic zone, thick fibrils in the resting zone or permanent cartilage, and a mixture of thin and thick fibrils in the proliferative zone. Tissues were examined by immunoelectron microscopy with antipeptide antibodies to the carboxyl telopeptide and to the amino terminal non-triple-helical domains of alpha 1 (XI). These studies showed that (a) both epitopes of Type XI collagen were readily accessible to antibodies at the fibrillar surface, (b) Type XI collagen was associated predominantly with fibrils < 25 nm in diameter, (c) Type XI collagen was not found in thick fibrils even after disruption with chaotropic agents, and (d) collagen Types II and IX were associated with fibrils of all sizes. These studies were extended to human newborn epiphyseal cartilage and to fetal calf cartilage, with the same result.

Growth plate pathology in feline mucopolysaccharidosis VI

The mucopolysaccharidoses (MPS) are a family of lysosomal storage diseases that result from the accumulation of partially catabolized glycosaminoglycans (GAGs) within lysosomes. A characteristic of most affected individuals is radiographic evidence of symmetrical epiphyseal dysplasia, with short stature and degenerative joint disease. Although there is evidence of epiphyseal dysfunction, little is known of the changes that occur at the morphological level. The growth plate of the femoral head was studied by light and electron microscopy in five cats with MPS VI (Maroteaux-Lamy syndrome, arylsulfatase B deficiency) and 12 normal cats. Compared with the normals, the MPS VI cat growth plates exhibited poorly organized proliferative zones, an almost total loss of column formation in the hypertrophic zone, an uneven chondro-osseous junction, a disorganized calcifying cartilage zone, and abnormal or reduced numbers of osteoclasts. By electron microscopy, the cytoplasm of affected cat chondrocytes was filled with membrane-bound vacuoles. Together these findings indicate that the MPS diseases cause major changes in growth plate structure and function.

Ultrastructural studies of the epiphyseal plate of chicks fed a vitamin D-deficient and low-calcium diet

The epiphyseal plates of rachitic chicks fed a vitamin D-deficient and low-calcium diet were examined ultrastructurally at 4, 7, 14, 18 and 21 days old. On and after 14 days, changes were observed. Chondrocytes in resting, proliferating and maturing zones commonly showed a reduction in cell organelles, suggestive of a decrease in synthetic activity of matrix components. In addition, the resting zone cells had numerous intracytoplasmic microfilaments. Mitotic figures were present but not increased in number in the proliferating zone. Autolysosome-like dense bodies in the chondrocytes and clusters of degenerative and necrotic chondrocytes were observed in the proliferating and maturing zones. In the calcifying zone of the rachitic epiphyseal plate, initial calcification was observed characterized by the deposition of apatite crystals in matrix vesicles and the formation of spherical crystal clusters. The crystal clusters were separated from each other by collagen fibrils on which only small amounts of apatite crystals had been deposited. In this zone, hypertrophic chondrocytes responsible for the initial calcification were seen, but not the stellate chondrocytes responsible for the progression of matrix calcification after its initiation. These findings suggest that accumulation of the proliferating and maturing zone cells as a characteristic lesion of the epiphyseal plate in rachitic chicks is caused by a disturbance of cell maturation. Such disturbance in the calcifying zone may explain the progression of the defect in matrix calcification.

The septoclast, a cathepsin B-rich cell involved in the resorption of growth plate cartilage

At the transition between growth plate cartilage and the endochondral bone region, transverse septa are being eroded to allow the advance of invasive capillaries. To find out whether resorption is due to proteinase activity, tissue sections prepared from the growth plate/metaphyseal interface of young rats were immunostained with antibodies to the cysteine proteinase cathepsin B. Intense staining was found in a cell that is associated with the growing portion of the invasive capillaries and extends between them and the transverse septum. This cell has a single nucleus, actively synthesizes protein, and shows two other characteristic features: the cytoplasm is packed with multivesicular and dense bodies rich in cathepsin B, and the cell apex ends in a ruffled border extending into the transverse septum and signs of dissociated extracellular matrix. Even though the ruffled border resembles that of the osteoclast, the cell was not immunostained by a monoclonal antibody that recognizes a 97 KD protein known as ED1 which characterizes rat osteoclasts, monocytes, and macrophages. Therefore, this distinctive cell produces the proteinase cathepsin B and appears to be involved in the resorption of the transverse septum. The cell has been named the "septoclast."

Ultrastructural changes of chondrocytes of growth plates of young broiler chickens predisposed to tibial dyschondroplasia

Growth plates from the proximal tibiotarsal bones of growing broiler chickens from lines exhibiting high and low incidences of tibial dyschondroplasia (TD) were compared for ultrastructural differences. Samples of growth plates were collected from five chickens of each line at 2, 7, 14, and 21 d of age. Each sample was prepared for ultrastructural evaluation after initial fixation in 2.5% glutaraldehyde and secondary fixation in 1% osmium tetroxide, each containing .7% ruthenium hexamine trichloride. No gross TD lesions were observed. The ultrastructure of growth plates of birds at 2, 7, and 14 d of age was similar between the groups. However, at 21 d of age, chondrocytes in the prehypertrophic zone of the high incidence group contained large lipid inclusions and vesiculated and disarranged stacks of rough endoplasmic reticulum with greatly dilated cisternae. Apoptotic and necrotic chondrocytes were present in the hypertrophic zone. Nuclei of apoptotic chondrocytes contained crescentic caps of condensed chromatin. The cytoplasm contained short segments of dilated granular endoplasmic reticulum, lipid droplets, and vacuoles. Necrotic chondrocytes appeared as amorphous masses with pyknotic and karyorrhectic nuclei. These changes were not observed in chondrocytes of the low TD incidence group and have not been previously reported in nonthickened growth plate cartilage.

Ultrastructural and histochemical studies of the epiphyseal plate in normal chicks

BACKGROUND

Chondrocytes in the epiphyseal plate undergo a series of well-defined stages, each stage containing a morphologically homogeneous cell population. However, biochemical studies show that there are some functionally heterogeneous cell types in the calcifying zone of the chick epiphyseal plate.

METHODS

We studied the sequence of chondrocytic maturation in the normal chick epiphyseal plate ultrastructurally and histochemically. Chondrocytes in the calcifying zone were of three distinct types, the appearance of each cell type being closely related to the stage of matrix calcification.

RESULTS

Clear cells were observed in the upper calcifying region, stellate cells appeared in the middle calcifying region, and hypertrophic clear cells appeared in the lower calcifying region. Rough endoplasmic reticulum (RER) and lysosome-rich cells were found, these being limited to the outermost layers of the calcifying zone and containing ACPase-positive products. Osteoclasts were attached to the matrix near the RER and lysosome-rich cells in the poorly calcified regions.

CONCLUSION

We hypothesized that each cell type played a different role in the initiation, progression, and maintenance of cartilage calcification. RER and lysosome-rich cells may be responsible for the resorption of uncalcified cartilage matrix, this resulting in induction of the osteoclastic resorption of the calcified matrix. In addition, the fate of the chondrocytes was twofold: hypertrophic clear cells died, while the RER and lysosome-rich cells survived, suggesting that these cells were transformed into osteogenic cells.

Zonal variations of types II, IX and XI collagen mRNAs in rat epiphyseal cartilage chondrocytes: quantitative evaluation of in situ hybridization by image analysis of radioautography

The spatial-temporal distribution of the mRNAs for type IX and type XI collagens were compared to that of type II collagen mRNA in the tibial epiphyseal plate cartilage of normal growing rats. The mRNAs were detected by in situ hybridization with radio-labelled specific probes and visualized by radioautography. The areas covered by the resulting silver grains were quantified by computer assisted image analysis. The areas in chondrocytes of each zone of the epiphyseal plate cartilage, which correspond to the stages of chondrocyte development and function were determined. Types II, IX and XI mRNAs were present to some extent in chondrocytes of all zones. The distributions of type II and type IX collagen mRNAs were similar with the highest concentrations in the proliferative zone, and the lowest in the resting and calcifying zones chondrocytes. In contrast, type XI collagen mRNA had a different distribution, with the lowest concentration in the resting zone chondrocytes and a significant decrease in the calcifying zone chondrocytes. These patterns correlates with the changes in chondrocyte function, and may reflect the roles of the type IX and type XI collagens. The data show that computer assisted image analysis of in situ hybridization radioautographic images is a precise, rapid tool for analysing differences in gene expression.

Morphological and biochemical characterization of mineralizing primary cultures of avian growth plate chondrocytes: evidence for cellular processing of Ca2+ and Pi prior to matrix mineralization

Advances in the culture of mineralizing growth plate chondrocytes provided an opportunity to study endochondral calcification under controlled conditions. Here we report that these cultures synthesize large amounts of proteins characteristically associated with mineralization: type II and X collagens, sulfated proteoglycans, alkaline phosphatase, and the bone-related proteins, osteonectin and osteopontin. Certain chondrocytes appeared to accumulate large amounts of Ca2+ and Pi during the mineralization process: laser confocal imaging revealed high levels of intracellular Ca2+ in their periphery and X-ray microanalytical mapping revealed the presence of many Ca(2+)- and Pi-rich cell surface structures ranging from filamentous processes 0.14 +/- 0.02 microns by 0.5-2.0 microns, to spherical globules 0.70 +/- 0.27 microns in diameter. Removal of organic matter with alkaline sodium hypochlorite revealed numerous deposits of globular (0.77 +/- 0.19 micron) mineral (calcospherites) in the lacunae around these cells. The size and spatial distribution of these mineral deposits closely corresponded to the Ca(2+)-rich cell surface blebs. The globular mineral progressively transformed into clusters of crystallites. Taken with earlier studies, these findings indicate that cellular uptake of Ca2+ and Pi leads to formation of complexes of amorphous calcium phosphate, membrane lipids, and proteins that are released as cell surface blebs analogous to matrix vesicles. These structures initiate development of crystalline mineral. Thus, the current findings support the concept that the peripheral intracellular accumulation of Ca2+ and Pi is directly involved in endochondral calcification.

Production of angiogenesis inhibitors and stimulators is modulated by cultured growth plate chondrocytes during in vitro differentiation: dependence on extracellular matrix assembly

Secretion of angiogenesis inhibitors and stimulators is modulated during in vitro differentiation of embryonic chick growth plate chondrocytes. Supernatants from dedifferentiated cells undergoing maturation to hypertrophic chondrocytes in suspension progressively inhibited vascular cell random migration and invasion of basement membrane matrix by endothelial cells. Maximal inhibition was exhibited by conditioned medium from hypertrophic chondrocytes. The same medium also repressed vascular cell migration induced by highly angiogenic Kaposi's sarcoma cell supernatants and prevented formation of an anastomosed network of tube-like structures by endothelial cells plated on matrigel. On the contrary, when the suspension culture of hypertrophic chondrocytes was supplemented with ascorbic acid, a condition leading to the formation of a mineralized tissue similar to calcified cartilage, a dramatic switch to production of angiogenic activity was observed. Medium conditioned by osteoblast-like cells derived from hypertrophic chondrocytes also induced vascular cell migration and invasion of basement membrane matrix. The presence of angiogenic activity in the conditioned medium was assessed also by an in vivo assay in mice using reconstituted basement membrane associated with heparin. Therefore, interactions of chondrocytes with their extracellular matrix are an absolute requirement for the expression of angiogenic activities by hypertrophic chondrocytes at late developmental stages.

An ultrastructural, microanalytical, and spectroscopic study of bone from a transgenic mouse with a COL1.A1 pro-alpha-1 mutation

A line of transgenic mice have been investigated that expressed moderate levels of an internally deleted human gene for the pro alpha (I) chain of type I procollagen. These mice expressed the gene at approximately 50% that of the endogenous gene. The gene construct was modeled after a sporadic in-frame deletion of the human gene that produced a lethal variant of osteogenesis imperfecta by causing biosynthesis of shortened pro alpha (I) chains. Periera et al. (1993) reported extensive fracturing in these mice with femurs that were shorter in length and bone that had decreased ash weight, mineral, and collagen content. These workers demonstrated an increased brittleness in bone using biomechanical measurements. The functional consequences of these mutant genes were examined in both transgenic and in normal littermate mice to determine if a valid model at the ultrastructural and analytical level had been produced for OI. X-ray microanalysis of bone mineral demonstrated a significantly lower calcium-to-phosphorus (Ca/P) molar ratio in transgenic mouse bone than in normal littermates; this was a feature of human OI bone. Fourier transform infrared spectroscopy confirmed that the mineral present was apatitic in nature despite the lower Ca/P molar ratio. Alizarin red skeletal staining showed the presence of multiple fracture calluses on the ribs and on the long bones of some of the transgenic mice, this was not seen on normal littermates. No light microscopic differences were observed between normal and transgenic mice; however, many ultrastructural correlates with human OI were observed in the transmission electron microscope. Anomalous fibrils associated with type I collagen, and an amorphous calcified material was observed lining the cartilage, extending beyond the lamina limitans in young transgenic mice.

Normal long bone growth and development in type X collagen-null mice

To investigate the role of type X collagen in skeletal development, we have generated type X collagen-null mice. Surprisingly, mice without type X collagen were viable and fertile and had no gross abnormalities in long bone growth or development. No differences were detected between the type X collagen-null mice and controls when growth plates of both newborn and 3-week old mice were examined by histology and by immunostaining for extracellular matrix components of bone including osteopontin, osteocalcin and type II collagen. Our results suggest that type X collagen is not required for long bone development. However, mice and humans with dominant acting type X collagen mutations have bone abnormalities, suggesting that only the presence of abnormal type X collagen can modify bone growth and development.

Ultrastructure of hypertrophic cartilage: histochemical procedures compared with high pressure freezing and freeze substitution

The effect of cationic dyes on the ultrastructure of hypertrophic cartilage was compared with results obtained with modern cryotechniques in studies on rat epiphyseal growth plate. Addition of alcian blue, acridine orange, cupromeronic blue, ruthenium hexamine trichloride, ruthenium red, or safranin O to conventional glutaraldehyde/osmium tetroxide fixatives to a large extent resulted in prevention of chondrocyte shrinkage except for alcian blue which showed poor tissue penetration. The fine structure of the matrix in pericellular and territorial compartments appeared very coarse with areas of high contrast in tissue exposed to fixatives containing cationic dyes. This indicates structural collapse and precipitation of electron-dense material, a pattern clearly differing from that observed in specimens prepared by the cryotechniques. The dyes giving a pattern most similar to that seen after high pressure freezing, freeze substitution, and low temperature embedding were acridine orange and safranin O. It is concluded that studies of matrix ultrastructure down to the molecular level necessitate the application of cryotechniques.

Chondrodysplasia in five Great Pyrenees

Five disproportionate, short-limbed, short-trunked (dwarf) Great Pyrenees pups were examined. The mode of inheritance was compatible with a simple autosomal recessive trait, and skeletal radiography revealed flaring of the metaphyses of all long bones and the costochondral junctions of the ribs. Vertebral bodies were poorly ossified and short, and had a beak-like extension on the caudal metaphyseal margin. Vertebral body end-plates were thin and concave, and ossification was abnormal. Three of the 5 dogs were deaf, and 1 had testicular atrophy. Ocular examinations did not reveal any abnormalities. Histologic examination of the growth plates revealed disorganized chondrocyte columns, and chondrocytes appeared to have undergone degenerative changes in the zone of chondrocyte proliferation. Transmission electron micrography of growth plate chondrocytes revealed dilated profiles of rough endoplasmic reticulum.

The immature goat as an animal model for Legg-Calvé-Perthes disease

Legg-Calvé-Perthes disease (LCPD) results from avascular necrosis of the capital femoral epiphysis in growing children. This disease often yields a significant deformity of the proximal femur, which may result in osteoarthritis. Its cause is unknown, although extensive radiographic, clinical, and histologic evaluations have been performed. Attempts at developing an animal model for LCPD have been unsuccessful. Previous models have been based predominantly on determining the vascular etiology of the disease. There is a need for an animal model that mimics the growth pattern of the proximal femur seen in LCPD. Such a model would allow for the development and testing of new treatments. Thus far, no treatment strategy has been completely successful. A study involving graphic analysis of radiographs found that arrested anterolateral physeal growth with continued or accelerated perichondrial ring and posteromedial epiphyseal growth would account for the most severe morphologic changes observed in the femoral heads of patients with LCPD. A surgical procedure was performed to ablate the capital femoral physis in goat kids in an attempt to mimic the changes noted in this study. The procedure was evaluated with radiographs, gross specimens, and histopathologic slides. Graphic analysis of the radiographs revealed changes in the shape of the operated femoral head compared with the unoperated femoral head. While bone, fibrous, and fibrocartilaginous bridges were histologically observed across the physis, the resultant deformities did not mimic the changes identified in the graphic analysis study, perhaps because of inconsistencies in the surgical ablative techniques, which will require further modification. This study provides the basis for further research to develop a successful model.

Growth plate matrix vesicle biogenesis. The role of intracellular calcium

Isolated bovine growth plate chondrocytes suspended in a synthetic cartilage buffer containing 10(-7) to 10(-3) M concentrations of calcium were treated with 1 microM ionomycin to induce changes in intracellular ionized calcium as measured by Fura-2 fluorescence. An increase in intracellular calcium of 10(-6) to 10(-4) M calcium resulted in the secretion of vesicles that contain alkaline phosphatase. An increase in intracellular calcium above 10(-4) M resulted in nonspecific cell fragmentation because of cell damage. Electron microscopy of the secreted vesicles demonstrated that their structure resembled matrix vesicles. Isolated cells treated with ionomycin (synthetic cartilage buffer with 10(-6) M calcium) demonstrated intact cell membranes, normal intracellular architecture, and numerous plasma membrane processes undergoing vesiculation. The plasma membrane of these isolated cells and the isolated vesicles demonstrated a positive stain for alkaline phosphatase. Fresh growth plate tissue demonstrated plasma membrane staining for alkaline phosphatase at the lower proliferative and upper hypertrophic cell zone, and an increasing number of alkaline phosphatase stained matrix vesicles in the matrix of the lower hypertrophic cell zone. The data indicate that an increase in intracellular calcium in the lower proliferative and hypertrophic cell zones of the growth plate induces the secretion of plasma membrane derived matrix vesicles.

Mechanism of longitudinal bone growth and its regulation by growth plate chondrocytes

Growth plate chondrocytes play a pivotal role in promoting longitudinal bone growth. The current review represents a brief survey of the phenomena involved in this process at the cellular level; it delineates the contributions made by various activities during the course of the chondrocyte life cycle, notably proliferation and hypertrophy, and illustrates how the relative contributions may be modulated according to the particular needs of an organism at critical phases of growth. The cellular mechanisms by which a few well characterized growth-promoting substances exert their influences are discussed in the light of recent findings pertaining to epiphyseal plate chondrocytes in vivo.

Image analysis of the extracellular matrix

The epiphyseal growth plate and articular cartilage matrices were preserved by slam freezing and freeze substitution to optimally retain the native organization for both cellular and matrix components. These specimens were stained and examined using conventional electron microscopic methods. The highly integrated, proteoglycan-rich matrices were examined by computer image analysis using such parameters as distribution, connectivity, orientation, and a variety of morphometric analyses. Also, different aspects of electron tomography and 3D rendering of matrix vesicles and their associated mineral deposits from epiphyseal growth plates and turkey leg tendons are presented.

[Experimental studies in effect of limb lengthening using metaphyseal osteotomy on epiphyseal growth plate]

The experimental results in effect of limb lengthening using metaphyseal osteotomy at proximal tibia on epiphyseal growth plate in 28 young sheep were reported. Radiographic, microscopic and electronmicroscopic findings showed that lengthening of 7.5% the growth plate resulted in narrowing in varying degrees in one third of the sheep with a lengthening rate of 0.25 mm every six hours. Narrowing of epiphyseal growth plate was seen in half of sheep with lengthening of 10% and in all sheep with lengthening of 12.5% and 15%. Degeneration and necrosis of chondrocytes happened in one sheep of lengthening of 15%. Thickening of growth plate was seen in all sheep of the control group. Our conclusion from this series of experimental studies was that depression caused by compression stress effect in limb lengthening of less than 7.5% on epiphyseal growth plate is minimal.

Mutation in the COL2A1 gene in a patient with hypochondrogenesis. Expression of mutated COL2A1 gene is accompanied by expression of genes for type I procollagen in chondrocytes

A new dominant mutation in the COL2A1 gene was found in a 38-week-old fetus with hypochondrogenesis. Denaturing gradient gel electrophoresis was used to analyze all 44 exons coding for the triple-helical domain of COL2A1 gene and the corresponding exon-intron boundaries. The technique detected a new sequence variation in exon 35. Sequencing of exon 35 demonstrated a single base mutation that converted the codon for glycine at position 604 to a codon for alanine. Electrophoresis of pepsin-digested collagen extracted from the diseased cartilage showed a doublet band of the alpha 1(II) chain of type II collagen and the presence of alpha 1(I) and alpha 2(I) chains of type I collagen. Two-dimensional analysis of cyanogen bromide peptides from the type II collagen revealed post-translational overmodification of peptides CB12, CB11, CB8, and CB10.5, whereas peptide CB9.7 migrated normally. Microscopic examination of cartilage showed that the mutation altered the organization of the growth plate. Also, articular chondrocytes contained large cisternae of rough endoplasmic reticulum. The density of the extracellular matrix was reduced, and the intensity of the staining with an antibody to type II collagen was diminished. In contrast, a significant staining with an antibody to type I collagen was observed. In situ hybridization with cRNA probes revealed a significant level of alpha 1(I) mRNA in the cytoplasm of the patient's chondrocytes. The signal for alpha 1(II) mRNA was about the same in control samples. The results indicated, therefore, that the genes for both type I and type II procollagens were simultaneously expressed in chondrocytes from the patient.

Morphological investigation of epiphyseal cartilage after glutaraldehyde-malachite green fixation

The glutaraldehyde-malachite green (GlMG) fixation has been reported to allow preservation of tissue lipids, mainly acidic phospholipids, that in the absence of malachite green are lost in the glutaraldehyde solution. This method has been used for the morphological study of calcifying epiphyseal cartilage, with particular reference to the presence of lipids in chondrocytes, cartilage matrix and calcification nodules. Both light and electron microscopy show that after GlMG-osmium fixation the chondrocytes contain cytoplasmic dense bodies. These have the same morphology as those found with the same method in other cells and tissues and considered to be lipid globules. However, they are not modified by treatment of GlMG-fixed specimens with chloroform-methanol. Formic acid decalcification induces partial solubilization of these bodies, whereas EDTA decalcification leaves them unchanged. They are found also in the pericellular space and in calcifying matrix. GlMG fixation improves the preservation of the crystal-associated organic structures (crystal ghosts) when the cartilage is decalcified before embedding. When these structures are demonstrated by the post-embedding decalcification and staining method, their morphology is similar to that found in controls. Moreover, GlMG fixation demonstrates the presence of typical rod-like, beaded structures, probably corresponding to proteoglycans, on the outer surface of the chondrocyte membrane. Although composition and role in calcification of the dense bodies remain uncertain, the important physiological implications that can be drawn from their presence in chondrocytes and cartilage matrix require further investigation.

Achondrogenesis type IB (Fraccaro): study of collagen in the tissue and in chondrocytes cultured in agarose

A lethal chondrodysplasia characterized by extreme micromelia was diagnosed by ultrasound examination in two sibs whose nonconsanguineous parents were healthy. Radiographic and histopathologic data indicated that the two foetuses (18 and 21 weeks old) had achondrogenesis type IB (Fraccaro). Quantitation of total collagen extractable from dried cartilage samples demonstrated a 50% decrease when compared to an age-related control. This decrease was essentially related to type II collagen. Nevertheless, the alpha chains and the CB peptides of type II collagen had a normal electrophoretic mobility. A significant amount of collagen type I was also detected. The electrophoretic pattern of collagens type IX and XI did not differ significantly from control sample. The extracellular matrix elaborated by patient chondrocytes cultured in agarose for 10-12 days, contained less collagen type II than normal cells. Labelling with 14C-proline of cultured cells showed the presence of procollagen and type II collagen chains with a normal electrophoretic mobility, but an alpha 2(I) chain was detectable in the patient material, indicating the presence of collagen type I which supported the tissue findings. The significance of the type II collagen reduction in the patient's cartilage is unclear but it is unlikely to be the primary defect in achondrogenesis type I.

[Torsional strength of the epiphyseal plate and fracture patterns with aging (three-dimensional analysis with SEM)]

The tortional strength and fracture patterns of the proximal tibial epiphyseal plate in 4-, 8-, and 12-week-old rabbits were investigated (n = 11, 13 and 8, respectively). The strength was measured with a torsional machine, and then maximum torque, angular deformation and energy absorption were calculated from the load-deformation curve. The rotation axis was aligned to the long axis of the specimen, and a holding method for the specimen was devised to minimize positional error. The fracture surfaces were then examined both histologically and with three-dimensional scanning electron microscopy (SEM). The torsion test showed that maximum torque and energy absorption with the exception of angular deformation increased significantly with age. Histological examination by light microscope of sagittal sections passing through the midline of fragments revealed that epiphysiolysis commonly occurred between the calcification layer and the metaphysis in the anterior portion, as well as between the transitional part of the palisading layer and the hypertrophying layer in the posterior portion. These results indicate that epiphysiolysis is most likely to occur between adjacent layers with the greatest structural differences. Three-dimensional SEM micrographs revealed that fracture patterns became more varied as the torsional strength of the epiphyseal plate was increased with age. Additionally, SEM revealed that the density of the collagen network in the cartilaginous matrix surrounding cells in the proliferating layer also increased with age. One of the reasons for the increase in torsional strength is related to a morphological change such as the development of three-dimensional wave-like structure of epiphyseal plate. Another reason appears to be a qualitative change such as the variety of the fracture patterns and an increase in the collagen network density of the cartilaginous matrix with age.

A comparative ultrastructural study of the mitotic chondrogenic cells in the mandibular condyle and tibial growth plate of the rat

Mitotic cells in the rat mandibular condyle (proliferative mandibular cells) were compared with mitotic cells in the rat tibial growth plate (proliferative tibial cells). In the tibial proliferative cells, the rough endoplasmic reticulum mostly became vacuolated during the latter stage of mitosis, whereas the rough endoplasmic reticulum of the proliferative mandibular cells rarely underwent disorganization. Further, a wide area remained outside of the mitotic spindle in the proliferative tibial cells at the metaphase, while only a narrow area remained in the proliferative mandibular cells. This finding might account for the difference of disorganization of the rough endoplasmic reticulum.

Ultrastructural organization of type XI collagen in fetal bovine epiphyseal cartilage

Type XI collagen was localized with polyclonal antibodies specific for alpha 1 (XI) and alpha 2 (XI) chains in the resting zone of epiphyseal cartilage from calf fetuses. The immunofluorescence technique was used on sections of cartilage, and the immunogold labelling technique for electron microscopy on fibrils isolated from cartilage and, for the first time, in situ on blocks of cartilage fractured in liquid nitrogen. Immunofluorescence showed that without pepsin treatment the staining of type XI collagen was restricted to the pericellular zones; after pepsin treatment, the staining was co-distributed with that of type II collagen. Immunoelectron microscopy performed on isolated fibrils and on cartilage blocks showed that after disruption of fibrils with pepsin, type XI collagen was labelled on small filaments on the fibrils. When the fibrils were not disrupted, labelling was observed in situ only at the ends of the fibrils or on cross-sections of some fibrils. These results indicate that type XI collagen is located inside type II collagen fibrils in fetal bovine epiphyseal cartilage, as already postulated for embryonic chicken sterna.